This invention relates to a battery operated power supply for an electroluminescent (EL) lamp and, in particular, to an inverter using SCR's as low side switches in an H-bridge.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage, the current through the EL lamp ceases, and the lamp stops producing light.
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by an inverter that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the EL lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.
The prior art discloses several types of inverters including an inductive boost circuit having an inductor in series with a switching transistor. Current through the inductor causes energy to be stored in a magnetic field around the inductor. When the current is abruptly shut off, the induced magnetic field collapses, producing a pulse of high voltage. The voltage across the inductor is proportional to L.cndot..delta.i/.delta.t. Thus, a low voltage at high current is converted into a high voltage at low current. The voltage on the lamp is pumped up by a series of high voltage pulses from the inverter.
The direct current produced by the inverter must be converted into an alternating current in order to power an EL lamp. U.S. Pat. No. 4,527,096 (Kindlmann) discloses a switching bridge, known as an H-bridge, to alternate the current through the lamp. The bridge changes the polarity of the current through the lamp at a low frequency (200-1000 hertz). In an H-bridge, the current paths through the transistors correspond to the current paths through the diodes in a full wave bridge rectifier. That is, the bridge has an AC diagonal, coupled to an EL lamp, and a DC diagonal, coupled to a boost circuit. The bridge operates like a double pole, double throw switch, as illustrated in FIG. 1 to produce an alternating current through the EL lamp. The transistors coupled to the supply voltage are known as the "high side" switches and the transistors coupled to common are known as the "low side" switches.
The EL lamp market is very cost sensitive and, for inverters, cost is approximately proportional to the size of a semiconductor die. In a bridge circuit, the peak current determines the size (and cost) of the switching transistors. Cost is also a factor of the kind of transistor, of which there are several.
The high side switches of an H-bridge are typically either silicon controlled rectifiers (SCRs) or p-channel metal-oxide-semiconductor, field effect transistors (PMOS FETs). The :low side switches of an H-bridge are typically NPN-type bipolar junction transistors or NMOS FETs. An advantage of MOSFETs is negligible gate current. An advantage of an SCR is also negligible gate current. Another advantage of an SCR is that an SCR can be made substantially smaller than MOSFETs of equal current capacity. An SCR is also far less susceptible to failure due to electrostatic discharge and over voltage than a MOSFET.
Some commercially available inverters use NMOS low side switches. Others use NPN bipolar junction transistors as low side switches. Although smaller than FETs of comparable current capability, there are two important drawbacks to bipolar junction transistors, they require significant base current and they must be sized for peak lamp current, causing them to be substantially larger than the equivalent SCR.
In view of the foregoing, it is therefore an object of the invention to provide an improved H-bridge for driving EL lamps.
Another object of the invention is to reduce the cost of low power inverters for EL lamps.
A further object of the invention is to improve the efficiency of low cost inverters for EL lamps.
Another object of the invention is to reduce the current drawn by a battery power inverter for driving an EL lamp.